This invention relates to optical connectors and, more particularly, to ferrules used within the optical connectors to hold optical fibers.
Commercial optical fiber connector assemblies 100, such as shown in exploded view in FIG. 1, are used to couple optical fibers together so that light transiting from a bundle 102 of one or more fibers in one end 104 of the connector assembly 100 will pass through the connector assembly 100 to fibers or a device connected to the other end 106 of the connector assembly 100. A ferrule 108 is typically part of the connector 100 and is the part of the connector 100 into which the fibers 110 themselves are inserted before the ferrule 108 is inserted into the overall connector assembly 100 itself. The ferrule 108 holds the fiber(s) 110 in a precise position and ensures that when the connector assembly is attached to a mating connector assembly or some other device, the fibers of the connector assembly are held in consistent alignment.
In the multi-fiber connectors available today, such as shown in U.S. Pat. No. 5,214,730, most of the connections are for fiber arrays of between 2 and 12 fibers arranged in a single row (although some commercial 2xc3x9712 configurations are available). Those connectors are referred to by various names, depending upon who makes them. In 1xc3x972 arrays, connectors are referred to as ST, LC, MT-RJ connectors while the 1xc3x9712 and some 2xc3x9712 array connectors are referred to as MTP(copyright), MPO, MPX and SMC connectors, among others. In the 1xc3x9712 or 2xc3x9712 area, all of the various connectors use a common type of ferrule commercially available from, among others, US Conec Ltd. and Alcoa Fujikura Ltd. Moreover, in some cases, the ferrules used in the small array connectors (i.e. for less than 12 fibers) are form and fit compatible for use with the MTP, MPO, MPX and SMC connectors. In addition, other types of commercial connectors for small arrays of fibers (i.e. less than 12) are available or have been proposed, for example, as shown in U.S. Pat. No. 5,743,785.
FIG. 2 is an enlarged photograph, in perspective view, of a prior art 1xc3x9712 ferrule 200 having an outer dimensional shape for use in an MTP, MPO, MPX or SMC connector of the prior art. Such ferrules 200 are made by molding plastic or epoxy. For example, the 1xc3x9712 (shown) and similar 2xc3x9712 ferrule technology currently in commercial use is based upon molding and curing of a glass filled epoxy resin (a high-performance plastic) using a common molding technique called transfer molding.
There has been an increasing need among users in the fiber optic field for larger groups of fibers, so demand for single connectors to handle arrays of fibers in excess of 12 has been increasing as well. Today, ferrules 200 such as shown in FIG. 2 that are molded out of epoxies or plastics can be made to the necessary tolerances for small arrays of multimode fibers, on the order of one or two rows of up to 12 fibers each, but special care must be taken during fabrication. Plastic molding technology is very process sensitive and molds having the requisite precision for even small arrays are extremely difficult to make. Even so, yields tend to be poor due to the inherent manufacturing process errors that occur in plastics molding. Since the tolerances on these pieces must be very accurate (on the order of about 1 to 2 microns), high yield manufacture is difficult when the array size necessitates two rows and exceptionally difficult for more than two rows.
The overall ferrule volume is very small, since ferrules 200 for the above MTP, MPO, MPX or SMC connectors are about 2 mm (2000 microns) high, 6 mm wide and 8 mm deep, and have a face portion of at least 3 mm thick to support and hold optical fibers, so molding or machining of features into the face surface 202 of the ferrules through the face portion, in the number and size required to hold multiple optical fibers (which typically have about a 125 micron cladding diameter for both multimode fiber and single mode fiber and are spaced from each other on a center-to-center spacing (xe2x80x9cpitchxe2x80x9d) of 250 microns), is very difficult.
Additionally, making ferrules for larger arrays is made even more difficult because, as the holes approach the periphery of the ferrule, the structural integrity of the peripheral walls near the holes decreases. In addition, process variations during production cause parts to also have poor tolerance at the periphery. As a result, they become overly fragile, causing hole and in some cases component collapse and/or they have distortions or excess material that impedes or prevents fiber insertion and are too fragile to successfully attempt removal of any such material. The problem is that in molding plastic ferrules for holding higher multimode fiber counts in the same small area results in even less structural integrity for the molded piece.
Nevertheless, in an attempt to address the increasing industry need, companies have attempted to manufacture connectors for larger arrays using the techniques currently used to manufacture small array ferrules (i.e. ones with a single row of between two and 12 fiber holes) with little to no success. For example, one company is known to have made a 5xc3x9712 array ferrule and 5xc3x9716 array ferrule. One example of the 5xc3x9712 ferrule is shown in the photograph of FIG. 3 and both are described in Ohta et al., Two Dimensional Array Optical Fiber Connector, Fujikura Technical Review (2000). However, although not discussed in the article, applicants were informed that, in making those ferrules according to the prior art molding technique, they achieved such poor yields that the commercial cost of producing the pieces was deemed prohibitivexe2x80x94in that the problems encountered and extremely low yield would result in their being sold for some $500 each, if they could be sold at all. Moreover, the process was such that the molds for producing the pieces were destroyed in the process. As a result, they deemed arrays of that size (i.e. arrays of 5 rows) unmanufacturable using the molding processes then available. Other companies, when asked if they could provide similar large array ferrules, would not even attempt to do so, considering them unmanufacturable without even trying.
As described in the Ohta et al. paper, the ferrule also includes a row of guide grooves for each row of holes. In the ferrule of FIG. 3, the access way has been enlarged and the upper rows of guide grooves have been removed so that the holes for the fibers can be viewed through the access way of the ferrule.
FIGS. 4, 6 and 7 are further photographs of the 5xc3x9712 ferrule of FIG. 3 taken from different views.
FIG. 4 is a close-up photograph of the exposed row of guide grooves taken looking into the ferrule through an access way from the same angle as in FIG. 3. The purpose of the guide grooves is to facilitate fiber insertion and to support the fibers once inserted by effectively increasing the thickness of the face portion by up to an additional 1.5 mm or more.
FIG. 5 shows a simplified view of a portion 500 of a ferrule having a 3xc3x974 array of fiber holes 502 and guide grooves 504, similar to those used in the ferrule of FIG. 3. The rows are stepped, with the lowest row 506 being the longest, and each successively higher row 508, 510 being slightly shorter. Depending upon the particular ferrule the guide grooves are semi-cylindrical or xe2x80x9cVxe2x80x9d shaped in cross section. During manufacture, fibers are inserted into the guide grooves of the lowest row, followed by the next higher row, etc. until all the desired fibers have been inserted. As their name implies, the guide grooves guide or direct the fiber into the fiber holes of the ferrule.
FIG. 6 is a closer photograph of the ferrule holes in the ferrule of FIG. 3 taken looking into the ferrule at an angle through the access way. As noted above, some of the rows of guide grooves have been removed so that several rows of holes are exposed for viewing. As can be seen in the photographs of FIG. 4 and, more clearly in the photograph of FIG. 6, there is visible variation in the size and shape of the holes as well as the walls separating one hole from another. These variations are due to the problems noted above. Depending upon the particular defect, the hole variation can inhibit fiber insertion, affect the pitch, or affect the inserted fiber angle (relative to other inserted fibers)xe2x80x94all undesirable results. In addition, although these holes are clearly visible in FIG. 6, in actuality, the fiber holes would be obscured from view by the guide grooves. In addition, the presence of the guide grooves makes it difficult, if not impossible, to fix a partially blocked or collapsed hole without damaging the ferrule.
FIG. 7 is a photograph of the same holes taken looking into the ferrule of FIG. 3 through the rear end of the ferrule. As can clearly be seen in this photograph, there is visible variation in the size and shape of the holes as well as the walls separating one hole from another including marked differences in hole size, partially blocked or collapsed holes and variation in wall thickness between adjacent holes.
As such, the prior art has been forced to do without commercial connectors for such large arrays, because such arrays can not be reliably created, and ferrules for use in commercial connectors for still larger format arrays are still considered unmanufacturable or prohibitively difficult for those in the art to even attempt. Moreover, since single mode fibers have an even smaller core diameter than multimode fibers and hence can have a smaller overall diameter, molding or machining ferrules for use in present form factor commercial connectors that will accommodate large arrays of single mode fibers is currently, for all practical purposes, considered equally prohibitive if not impossiblexe2x80x94particularly on a cost effective commercially viable scale.
Thus, our attempts to find an entity that could mold a commercially available connector compatible plastic ferrule to accommodate an array of 5 rowsxc3x9712 fibers/row or any large format array (in terms of number of rows over two, irrespective of fibers per row) left us discouraged and, like those in the art seeking similar pieces, to the conclusion that such ferrules could either not be made on a commercially viable scale or could not be made at all.
Thus, despite the strong and growing need for ferrules that can be used for large arrays of fibers, and the attempts in the art to fulfill those needs, the art has not been able to successfully do so. Moreover, to the limited extent anyone has even been able to mold the above 5xc3x9712 or 5xc3x9716 plastic ferrules at all, the ability to consistently and reliably produce such ferrules to address the need in the art at all, let alone in commodity item quantities, is elusive.
We have discovered that, contrary to conventional wisdom and the teachings of the prior art, by fabricating a ferrule where the overall thickness of the forward portion of a ferrule (including any structures that facilitate fiber insertion) is less than the 3 mm (i.e. 3,000 microns) or more used in the prior art, a large format array ferrule can be formed using conventional molding techniques. Moreover, it can be done with commercially suitable, cost competitive, yields.
In addition, we have discovered that, by reducing the thickness of the forward portion to less than the 3 mm used in the prior art, such large format array ferrules can also be formed not only using transfer molding techniques, but also injection molding, casting, or powder metallurgy-type xe2x80x9cpressingxe2x80x9d techniques, as appropriate, for polymers (including thermoplastics, polyimides, curable resins, thermoset resins, etc.), ceramics and metals.
We have further discovered that, contrary to conventional wisdom and the teachings of the prior art, as a separate and significant matter, the guide grooves conventionally thought to be necessary for fiber guiding and/or support, can be dispensed with almost entirely, if not entirely, without unacceptably affecting ease of fiber insertion or fiber support so long as the overall thickness of the forward portion plus the guide grooves (as measured along the axis of a fiber hole) is less than 3000 microns.
The reduced thickness of the forward portion of a ferrule made according to the present invention, particularly when done in conjunction with the elimination, or virtual elimination, of the guide grooves, allows a commercial connector compatible large format array ferrule to be molded repeatably, in commercially suitable quantities, in a cost competitive manner using prior art techniques. Such inventive ferrules have better uniformity in size and shape of the holes and walls between them, in large formats, than could previously be made according to the prior art, without reducing the structural integrity of the ferrule near the face to a point of concern.
A ferrule in accordance with the present invention therefore represents a significant and valuable improvement in the art and addresses a significant need in the art.